1. Field of the Invention
The present invention relates to a radio communication system in which out of a plurality of communication units which perform radio communication with each other, one operates in a master mode while the others operate in a slave mode, and each of the communication units is operated by a battery.
2. Description of the Related Art
There is disclosed in the microfilm of Japanese Utility Model Application Laid-Open No. S62-155535 a radio communication system in which, in order to allow conversation between riders (driver and passenger) on two-wheeled vehicles, a helmet of each rider is provided with a communication system, that is, intercommunication system, comprising a speaker, a microphone and a communication unit, thereby allowing a direct conversation between the riders. In addition, a technique which utilizes Bluetooth as radio communication standards of the interconnection system is disclosed in the Japanese Patent Application Laid-Open No 2001-155534.
When a radio communication unit is mounted on a helmet of each rider and power source of the communication unit is supplied from the vehicle, a wiring is required so as to connect the vehicle to the helmet of the rider and therefore, a small battery like a dry cell is preferably used as power source. In such a situation, in order to achieve a longer battery operating time, a technique for reducing power consumption of the radio communication unit is of importance.
In the radio communication system, when a time during which communication is not performed exceeds a predetermined time, each communication unit is changed into a power saving mode, called “sleep mode” or “standby mode”.
According to Bluetooth standards, among a plurality of communication units, one communication unit functions as a master and the others as slaves, which consist in a Personal Area Network called “piconet”. Also in this piconet, a power saving mode called “Sniff mode” is prepared.
In a normal mode other than Sniff mode, each slave is required to prepare for receiving from the master in all time slots on the ACL (Asynchronous Connection-Less) link. In the Sniffmode, as shown in FIG. 16, times lots (Sniffslots) specified at predetermined intervals called Sniff period (Tsniff) are prepared and packet transmission/reception is limited in these Sniff slots.
Slaves are allowed to keep in synchronization in piconet, even if they are in the Sniff mode, and have their own address information (BD_ADDR). When each slave receives a packet in a Sniff slot, it refers to an AM_ADDR registered in a packet header. When the packet is addressed to this slave, it continues to receive the packet. When the packet is not addressed to this slave, it stops receiving the packet to prepare for next Sniff slot. The above-mentioned Sniff slot does not have to be a single time slot (625μ second), but may be plural time slots (Nsniffattempt) as shown in FIG. 17.
Thus, since the communication units in the slave mode do not have to receive packets with all reception slots in the Sniff mode, it is possible to reduce power consumption. However, the master can not be changed into the Sniff mode.
For this reason, only a battery of the communication unit operating in the master mode is likely to be exhausted earlier than batteries of the other communication units operating in the slave mode. If the master is rendered inoperable by lack in the remaining battery life, the slaves become incapable of communicating irrespective of the remaining battery life.